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That, at least, is the calculation of the Water Footprint Network, a nonprofit foundation in the Netherlands that advocates for more sustainable, efficient and fair ways to use water.

Mesfin Mekonnen, a postdoctoral researcher at the University of Twente in the Netherlands, and a Water Footprint associate involved in compiling data, said via email that the 29-gallon figure was based on such factors as rainfall, irrigation and water used in cellars during winemaking.

In California vineyards and cellars, is 29 gallons of water to produce a single glass of wine a realistic estimate?

No, says Larry Williams, a professor in the Department of Viticulture and Enology at UC Davis who long has studied the water needs of vineyards. For one, the Dutch calculations, says Williams, don’t consider the much higher yields of California vines compared with vines of other grape-growing regions. His research indicates that California vineyards produce two to four times as much fruit as vineyards in Europe.

“The mean yield of wine grapes in Europe ... is around 1.8 tons per acre using data I’ve gleaned from research papers,” Williams says. “The mean chardonnay yields across California are 7.4 tons per acre.”

While higher yields in California may provide more wine for the water buck, no one knows just how much water is tapped to grow grapes and make wine here, though Williams, among others, can run calculations to estimate how much water a vine will use.

Any calculation of the wine trade’s use of water involves a look at two separate but related areas of exploitation – the vineyard is one, the winery another.

Getting a handle on water use in vineyards is laborious and highly technical, notes Williams. Among other things, it requires that evapotranspiration be measured (water that passes through the vine via transpiration plus water lost from the soil via evaporation) as well as how the plot is irrigated, if it is at all. (Williams estimates that about 90 percent of California’s vineyards are irrigated, though the amount can vary greatly.)

For eight years, Williams studied the water profile and water use of a chardonnay vineyard in the Carneros district of Napa Valley. One area of the vineyard was dry-farmed; that is, no water was applied. Another area was irrigated. The dry-farmed area used 213 gallons of water per vine per season, all from the soil, while the irrigated area used 295 gallons per vine (169 gallons from the soil, 126 gallons from irrigation).

Vines of the dry-farmed portion yielded 4.9 tons per acre, while vines on the irrigated portion produced 6.3 tons per acre. The upshot was that 14.2 gallons of water was needed in the dry-farmed block to produce a typical 4-ounce pour of wine, while 15.3 gallons of water was needed in the irrigated parcel to produce a 4-ounce pour of wine, totals far lower than the figure calculated by the Water Footprint Network.

Furthermore, notes Williams, “if one considers only the applied water amount for the irrigated treatment (in the above figures), then it took ... 6.5 gallons of irrigation water in the vineyard to produce a 4-ounce pour.”

His studies in other appellations with other grape varieties have come up with similar results. In Paso Robles, the amount of water applied to a vineyard of cabernet sauvignon ranged from 105 to 376 gallons per gallon of must, the unfermented juice of freshly squeezed grapes, not quite wine but close; that works out to a range of 3.3 to 11.8 gallons of water per 4-ounce glass of wine. In the San Joaquin Valley, the amount of water applied to a merlot vineyard ranged from 185 to 455 gallons per gallon of must, or roughly between 5.8 and 14.2 gallons of water per glass.

Several factors complicate the evaluation of water use by vineyard. In addition to whether and how a vineyard is irrigated, they include the spacing of rows, the nature of the rootstock, the type of trellising, the water-retention capability of the soil, whether water is needed for frost protection, and the temperatures, sunlight and even wind speed of the area in which the vineyard is cultivated.

In El Dorado County, Fair Play grape grower John Smith, a scientist before he founded the twin wineries Oakstone and Obscurity, which he subsequently sold, has been tracking how much applied water is used by his vines and how much wine they have been yielding. He’s found that each of his cabernet sauvignon vines uses about 289 gallons of irrigated water and yields about 1.3 gallons of wine. “This translates to 58 gallons of water per 750-milliliter bottle, or 9.6 gallons per 4-ounce pour,” Smith says.

He also farms a 5-acre stand of zinfandel that is completely dry-farmed. It produces between 2 1/2 and 3 tons of grapes to the acre, compared with 4 tons for his plot of cabernet sauvignon. “It all depends on how you farm, the variety, the trellising, the rootstock, the soil, the elevation, the insolation (solar radiation), the slope … and probably some other factors,” says Smith to illustrate the challenge farmers face in calculating their water use.

He predicts that if the drought persists and competition for water intensifies, more grape growers could switch to dry farming. “If there’s no water, there’s no option,” Smith says.

As to water that vintners use in their wineries, that total is comparably small, though it also can range widely. Vintners and people who study the trade agree that 2 to 6 gallons of water customarily is used in wineries alone for every gallon of wine that is made, though that total can be as low as half a gallon and as high as 20 per gallon of wine. Much of that water is for cleaning hoses, oak barrels, fermentation tanks and the like, with the variation due to such factors as the kind and number of vessels and the frequency with which wine is moved from one tank to another.

Smith figures that he and the new owners of Oakstone Winery, Steve and Liz Ryan, use no more than half a gallon of water in the cellar to produce a gallon of wine, or 2 ounces of water per 4-ounce glass of wine. In a typical year, he adds, 2,000 gallons of water would be reclaimed from chores in the winery and then distributed in the vineyard, a not-uncommon practice in California.

Some of Smith’s winemaking neighbors also keep a close eye on their water use. At Miraflores Winery of Pleasant Valley just north of Fair Play, winemaker Marco Cappelli says that between irrigation in the vineyard and sanitation practices in the winery, he uses nearly 9,000 gallons of water for every ton of grapes grown and processed, which works out to 12.5 gallons of water per 750-milliliter bottle of wine, or about 2.5 gallons of water per glass of wine.

Overall, California vintners, mindful of intensifying demands for water generally and the rising cost to secure adequate supplies, are looking for ways to cut their use and to capture and recirculate what they use.

A focal point in that effort is the new teaching and research winery at UC Davis, where the ultimate goal is to equalize the use of water with the output of wine; in other words, 1 gallon of the former per 1 gallon of the latter. “It’s been critical to us to build a winery that uses water over and over again. That’s the way to reduce our carbon footprint,” says Roger Boulton, the UC Davis professor of winemaking who played a pivotal role in the design and construction of the campus winery.

While the winery hasn’t yet achieved the 1:1 ratio of water to wine it seeks, it is making strides, says Charles “Chik” Brenneman, the campus winemaker. Between 2012 and 2013 alone, he notes, the winery reduced its use of water from 8 liters per liter of wine to 6 1/2 liters per liter of wine.

How did they do it? Mostly by relying on a dozen water meters installed throughout the facility instead of the customary one. This single measure helped faculty, students and staff gauge just how water was being used and to take steps to trim their consumption. “It’s education,” Brenneman says. “We read the meters daily and become more conscious and more careful in our use of water.”

Some California winemakers say they are beating or close to meeting the UC Davis standard. At its Paso Robles facility, J. Lohr Vineyards & Wines has cut its use of water to 1.2 gallons for each gallon of wine it makes. This savings is being accomplished via such steps as installing low-flow, high-pressure hoses for cleaning equipment, according to “Down to Earth,” a newly published Wine Institute book that surveys the sustainability programs of 15 of California’s grape growers and winemakers. (See story at left.)

So, while the claim that 29 gallons of water is needed to make a single glass of wine may apply to Europe, it doesn’t look to fit California. At the most, around 15 gallons of water per glass of wine may be realistic, and as little as 2 or 3 gallons of water per glass of wine has been obtained by some vintners.

More comforting, but you still may want to remove that silly bumper sticker.

Next week: A closer look at one winery’s successful effort to clean up its water act.

Sustainability

The California wine trade is rife with buzzwords whose definitions are elusive and unsettled – “biodynamic,” “organic” and “natural,” for example.

Another is “sustainable,” the term commonly invoked by farmers and vintners to encompass steps they are taking to assure that their businesses become and remain environmentally, economically and socially viable.

In their day-to-day operations, however, what does that mean? Soy-based inks for wine labels. Herds of sheep to control weeds. Cardboard instead of plastic foam for shipping containers. The composting of grape pomace. High-speed roll-up doors so that little cold air escapes as forklift drivers shuttle between winery and barrel room. Owl boxes.

Those are some of the measures that Janet Fletcher outlines in “Down to Earth: A Seasonal Tour of Sustainable Winegrowing in California” (Wine Institute, $40 255 pages), a brisk survey of 15 wineries frequently recognized for their inventive ways to maintain some sort of environmental, economic and social equilibrium. Her findings are illustrated liberally and brightly with the art of veteran wine-country photographer George Rose.

What’s more, Fletcher, a cook and food writer who has won three James Beard awards, concludes the text with recipes for such enticing dishes as grilled oysters with a creamy chipotle sauce, and a wild mushroom soup with Parmesan croutons. Pair with a fitting wine, or a glass of water.